1. Field of the Invention
The present invention relates to a general purpose X-ray tube for stereography, usable in the field of radiology and more particularly in that of radiodiagnosis.
2. Description of the Prior Art
The stereographic effect is obtained by radiographing the same subject from two sources, located independently in space, each defining an image or picture of the object seen at a different incidence. Stereoscopic viewing of this subject is obtained by regarding these two images in binocular vision, i.e. each of the images is regarded independently by each of the two eyes; the two retinal perceptions are conveyed by the optic nerve to the brain, which which merges them together and provides three-dimensional perception thereof.
The quality of this three-dimensional perception is related to the conditions of taking the pictures and to the conditions in which these pictures are viewed. Thus, in a first case, in which pictures are taken with an object in the vicinity or in contact with a film, determining a minimum enlargement, three-dimensional viewing reproducing this object under the best conditions is obtained when:
(a) a distance between the two sources is equal to the distance between the eyes, i.e. approximately 65 mm;
(b) when an "eyes-pictures" distance on viewing is equal to a "source-pictures" distance when the pictures are taken.
This poses a problem because, in practice, for better observation of details, the "eyes-pictures" distance on viewing is less than the "source-pictures" distance when the pictures are taken. Modifying the picture taking conditions, for example, effected in a first direction, is likely to re-establish the quality of this three-dimensional vision; but this modification must be exerted in a direction opposite the first one when, so as to obtain a larger enlargement, the object is moved away from the film for taking the pictures.